1. Field of the Invention:
This invention relates to the production of radiation curable microcapsular coating compositions. In particular, it relates to the production of microcapsules containing a hydrophobic liquid core by polycondensation polymerization of two wall-forming materials in a hydrophilic liquid continuous phase wherein the hydrophilic liquid comprises a polar radiation curable compound. In one embodiment of this invention, the encapsulated hydrophobic liquid contains a chromogenic material soluble in the encapsulated hydrophobic liquid. A dispersion of these microcapsules containing a chromogenic material can be coated on a substrate and cured by radiation to give a pressure-sensitive carbonless copy sheet having a transfer coating.
2. Prior Art:
The production of microcapsules containing an encapsulated oily (hydrophobic) liquid wherein the microcapsule walls are produced by a polycondensation reaction of polyisocyanate and a second wall-forming material is described in U.S. Pat. No. 3,796,669 to Kiritani et al. Both the polyisocyanate wall-forming material and the second wall-forming material are mixed with oily liquid. The mixed oily liquid is dispersed into an aqueous continuous phase and the temperature is raised to initiate the reaction on the surface of the oil drops to encapsulate the oil drops with the reaction product of the polyisocyanate and second wall-forming material. A catalyst for the reaction may also be added to the oily liquid.
A number of patents disclose the production of microcapsules using interfacial condensation polymerization of two or more reactants to form the microcapsule walls. Typical of these are:
U.S. Pat. No. 3,429,827 (1969) to Ruus PA1 U.S. Pat. No. 3,432,427 (1969) to Kan et al PA1 U.S. Pat. No. 3,464,926 (1969) to Vandegaer et al PA1 U.S. Pat. No. 3,492,380 (1970) to Santo et al PA1 U.S. Pat. No. 3,575,882 (1971) to Vandegaer et al PA1 U.S. Pat. No. 3,577,515 (1971) to Vandegaer. PA1 U.S. Pat. No. 3,607,776 (1971) to Santo et al PA1 U.S. Pat. No. 3,726,804 (1973) to Matsukawa et al PA1 U.S. Pat. No. 3,875,074 (1975) to Vassiliades et al PA1 U.S. Pat. No. 2,712,507 (1955) to Green PA1 U.S. Pat. No. 2,730,456 (1956) to Green et al. PA1 U.S. Pat. No. 3,455,721 (1969) to Phillips et al. PA1 U.S. Pat. No. 3,466,184 (1969) to Bowler et al. PA1 U.S. Pat. No. 3,672,935 (1972) to Miller et al. PA1 U.S. Pat. No. 3,720,623 (1973) to Cartmell et al. PA1 U.S. Pat. No. 3,551,235 (1970) to Bassemir et al. PA1 U.S. Pat. No. 3,551,246 (1970) to Bassemir et al. PA1 U.S. Pat. No. 3,551,311 (1970) to Nass et al. PA1 U.S. Pat. No. 3,558,387 (1971) to Bassemir et al. PA1 U.S. Pat. No. 3,661,614 (1972) to Bassemir et al. PA1 U.S. Pat. No. 3,720,534 (1973) to Macaulay et al. PA1 U.S. Pat. No. 3,754,966 (1973) to Newman et al. PA1 U.S. Pat. No. 3,772,062 (1973) to Shur et al. PA1 U.S. Pat. No. 3,772,171 (1973) to Savageau et al. PA1 U.S. Pat. No. 3,801,329 (1974) to Sandner et al. PA1 U.S. Pat. No. 3,819,496 (1974) to Roskott et al. PA1 U.S. Pat. No. 3,847,768 (1974) to Kagiya et al. PA1 U.S. Pat. No. 3,847,769 (1974) to Garratt et al. PA1 U.S. Pat. No. 3,242,051 (1966) to Hiestand et al. PA1 U.S. Pat. No. 3,265,630 (1966) to Jensen PA1 U.S. Pat. No. 3,405,071 (1968) to Reyes PA1 U.S. Pat. No. 4,021,364 (1977) to Speiser et al.
It is important to note that none of the above listed patents or U.S. Pat. No. 3,796,669 to Kiritani disclose the in situ preparation of the microcapsules in a radiation curable hydrophilic liquid containing a radiation curable polar compound.
Carbonless copy paper, briefly stated is a standard type of paper wherein during manufacture the backside of the paper substrate is coated with what is referred to as a CB or transfer coating, the CB coating containing one or more chromogenic materials, generally in capsular form. At the same time the front side of the paper substrate is coated during manufacture with what is referred to as a CF coating, which contains one or more chromogenic materials capable of producing a color with encapsulated CB chromogenic material. Both the chromogenic materials remain in the coatings on the respective back and front surfaces of the paper in substantially colorless form. This is true until the CB and CF coatings are brought into overlying relationship and sufficient pressure, as by a typewriter, is applied to rupture the CB coating to release the encapsulated chromogenic material. At this time the chromogenic material contacts the CF coating and reacts with the chromogenic material therein to form a colored image. Carbonless copy paper has proved to be exceptionally valuable image transfer media for a variety of reasons, only one of which is the fact that until a CB coating is placed next to a CF coating both the CB and CF coatings are in an inactive state as the coreactive elements are not in contact with one another until pressure is applied. Patents relating to carbonless copy paper products are:
A disadvantage of coated paper products such as carbonless transfer papers stems from the necessity of applying a liquid coating composition containing the color forming ingredients during the manufacture process. In the application of such coatings, volatile organic solvents are sometimes used which then in turn requires evaporation of excess solvent to dry the coating thus producing volatile solvent vapors. An alternate method of coating involves the application of the color forming ingredients in an aqueous slurry requiring removal of water by drying. Both methods suffer from serious disadvantages. In particular, the organic solvent coating method necessarily involves the production of generally volatile solvent vapors, creating both a health and fire hazard in the surrounding environment. When using an aqueous solvent system the large amounts of water must be evaporated since the microcapsule coatings currently used commercially generally comprise 60% to 85% water. This involves the expenditure of significant amounts of energy and further necessitates a separate drying step which requires the use of complex and expensive apparatus to continuously dry a substrate which has been coated with such aqueous coating compositions.
Radiation curable coating compositions and methods of producing these compositions are well known, although their use is not well known in the production of carbonless papers. In general, patents concerned with the production and application of liquid resin compositions containing no volatile solvent which are subsequently polymerized by free radical radiation to a solid film are:
These compositions generally also contain a pigment or dye. Such resin compositions are useful for protective coatings and fast drying inks. U.S. Pat. No. 3,754,966 describes the production of an ink releasing dry transfer element which can be used as a carbon paper or typewriter ribbon. Additionally it is known to use such radiation in the formation and hardening of microcapsules although none of these microcapsules are known to have utility in the carbonless paper environment. Patents known to use radiation in the preparation of microcapsules are:
It is significant to note that the encapsulated materials disclosed in any of the above patents do not include oil solutions of colorless dyes. It is particularly significant in view of the fact that the radiation cured coating of the instant invention must be compatible with the reaction of CB and CF chromogenic materials to form a color in order to have utility in the carbonless paper environment. Such color forming reactions are generally of a sensitive or delicate nature and are not generally compatible with the compositions found in the prior art. For instance, it is known that certain color precursors currently used in the commercial production of carbonless transfer sheets will undergo a color change when exposed to ultraviolet radiation.
The novel liquid coating compositions of this invention are dispersions of microcapsules having a hydrophobic core liquid in a radiation curable hydrophilic liquid. The hydrophilic liquid contains water which constitutes about 20% to about 60% by weight of the coating composition. The novel coating compositions are prepared by a novel process in which the microcapsules are formed in situ in the radiation curable hydrophilic liquid. The radiation curable hydrophilic liquid cures by exposure to give a tack-free film containing microcapsules thus omitting an additional drying step necessary for the prior art solvent or aqueous based microcapsular coating compositions. The cured continuous phase acts as a binder to adhere the microcapsules to a substrate. In a preferred embodiment, the microcapsules can contain a chromogenic material in the hydrophobic core liquid. Coating compositions containing such microcapsules may be used in the preparation of pressure-sensitive transfer papers.
The microcapsular coating compositions of the instant invention have a significantly lower water content than the microcapsular coating compositions currently used in the preparation of carbonless transfer papers. For example, prior art coating compositions containing gelatin are seldom applied at a water content below 80% (20% solids). In comparison with a microcapsular coating composition containing 40% water (60% solids) produced by the process of this invention, the amount of water necessary to be evaporated from the prior art compositions is 6 times the amount to be evaporated from the coating compositions of this invention. By using the coating compositions of this invention, a substantial reduction in heat energy used in drying can be realized. Another advantage is that by using available sources of ultraviolet radiation which also contains infra-red radiation such as the radiation from a mercury vapor lamp the microcapsular coating may be cured in one combined in one combined polymerization and drying step, thus eliminating the need for additional or separate drying.